HRSS recreation of F9, TWR question

[mafish]

Hello everyone!

Recently I've been messing around with HalfRSS mod (best decision ever!) and, of course, tried to recreate Flacon 9, both 1st and 2nd stage.I've found this sheethttps://docs.google.com/spreadsheets/d/1X9FUdHnowboCrjoWKpTM5OtLEqj26UWhF0UbEnON5_Q/edit?f=true&noheader=true&gid=18#gid=27

With pretty reasonable info about falcons parameters. I've scaled them down to be suitable, for HRSS, taking total delta-V as my base.
Parameter | Falcon 9  |  HRSS  | Mine
Total dV     |   9380      | ~7200 | 7038
1st st dV    |   3076      | ~2371 | 2369
2nd st dV   |   6388      | ~4925 | 4669

With all this, i struggle to reach orbit. it seems that my 2nd stage has not nearly enough TWR to push craft to orbit.
separation occurs somewhere between 40 - 50 km high (AP at 95-100km), and 2nd sdtage starts its burn right away. sadly no matter what pitch I set, 2nd stage doesn't even burn through half of its dV, until it falls back to atmosphere.

What might be the reason for this? Is Falcon9 data I based inaccurate? or its HRRS fault (longer to reach AP in RSS than in HRSS?), or maybe my gravity turn is made badly? 

Cheers

 

[Rocket In My Pocket]

Video games and real life aren't 1 to 1 compatible. Simple as that.

There are all sorts of factors, forces, and physics that are present in the real world that KSP doesn't even attempt to simulate. The things it does simulate; it simplifies.

Also, welcome to the forums!

Edited December 22, 2017 by Rocket In My Pocket

[Starman4308]

Would it be possible to upload your engine configurations somewhere where we could see them? From the data I can find, the Falcon 9 second stage should have an initial TWR of close to 1.0, which should be plenty to circularize on any reasonable ascent trajectory.

In more general terms: there are several possibilities. First, the Falcon 9 in particular uses a fairly steep ascent, something that should give the second stage plenty of time to circularize while "borrowing" upwards momentum from the first stage; it should also be well out of the thick atmosphere, meaning minimal drag losses and effectively vacuum engine performance.

This could be the case; you're staging into the second stage at a much lower altitude than what the real Falcon 9 does. Looking at the CRS-13 webcast, it seems to me that second-stage ignition is at 70 km altitude, with a vertical velocity of around 1 km/sec.

Second, if you accidentally copied the wrong number, such as the second-stage thrust in lbf instead of N, that could explain things.

Third, if you're using stock fairings, those can be pretty heavy, and may affect the dynamics of the vehicle.

Overall, my suspicion is that #2 is likely the major cause, since a 1.0 TWR is towards the upper end of second-stage TWRs, and should be far more than enough to circularize. With 1.0 TWR, you can literally point straight up and resist gravity.

[mafish]

Best results I got from this particular design was when I pitched up 10-15 degrees over velocity vector with "Surface" reference frame. It lets me push AP to ~230km and burn through all the fuel, but was really inefficient, leaving me with no fuel when I needed about 800dV to circularize.

On the 1st stage, I've used upscaled MainSail (with the TweakScale mod, to keep things simple), it gave me close-enough dV and TWR to the excel sheet I posted before, and for the second stage I've used standard Poodle, it kept stats pretty close to the sheet and was the simplest solution. 

Guess I will watch couple of launches yet again, this time doing some on altitude and velocity, and then maybe cheat-edit the Poodle to achieve 1.0 TWR

Thank you for your suggestions,

Cheers

 

[Starman4308]

Okay, are you using RealFuels or SMURFF? There are severe issues attempting to replicate real-world performance figures with stock equipment.

Stock tanks are vastly heavier than they should be, with a 9:1 full:empty mass ratio, whereas the Space Shuttle external tank hit something around 28:1.

Stock engines are vastly heavier than they should be, with execrable TWR. The Mammoth hits 27.18 vacuum TWR; the Merlin 1D has a TWR in excess of 150, the RS-25 a TWR of 66 (even burning very low-density hydrolox), etc.

If attempting to replicate anything burning methalox (liquid methane and liquid oxygen) or hydrolox (liquid hydrogen and liquid oxygen), you're also going to run into the issue that KSP specific impulses are calibrated towards those of RP-1/liquid oxygen rockets, whereas methalox goes up to ~370 s^-1, and hydrolox in excess of 450 s^-1.

All of this is because the stock solar system is a tiny toy relative to the real world, so Squad took a nerf to propulsion to make things even remotely difficult to achieve. If you're using HRSS, I'd suggest using either RealFuels (probably with the stockalike configs), or SMURFF with a configuration in between stock and its full RSS configuration.

Overall, my suspicion is that you're simply running head-first into the issue of trying to replicate a real-world rocket with underpowered stock parts.